Set xfs sector size into 512

By default mkfs.xfs sets sector size equal to
physical block size which makes it impossible to have
O_DIRECT mode. Enforcing xfs to use 512 byte size
fixes this problem but decreases fs performance a bit.

Change-Id: I4b38ef148c16baacfba12ed07534a3761b3cecf1
Related-Bug: #1316266